1. Field
This invention relates to a nitrogen-containing carbon alloy, a process of producing the carbon alloy, and a carbon catalyst containing the carbon alloy. More particularly, it relates to an N-containing carbon alloy obtained from a low-molecular N-containing crystalline organic compound, a process of producing the same, and a carbon catalyst containing the same.
2. Description of the Related Art
Catalysts based on noble metals, such as platinum and palladium, have been applied for their high oxygen reduction activity to, for example, solid polyelectrolyte fuel cells used in automobiles, and domestic power and heat supply systems. However, the noble metal-based catalysts are too costly to be used even more widely.
Research and development have therefore been invested into platinum-free catalysts or catalysts with greatly reduced use of platinum. For example, JP 2007-26746A proposes a catalyst having carbon alloy obtained by baking a polymer resulting from polymerization of a mixture of a precursor of a resin, such as polyfurfuryl alcohol, phenol formaldehyde or melamine resin, and a metal complex, such as cobalt phthalocyanine or iron phthalocyanine. Such a carbon alloy catalyst is known to have high oxygen reduction activity because it is obtained by uniformly dispersing the metal complex in a resin precursor followed by decomposing other than the metal, for example, phthalocyanine moiety so that the metal is uniformly distributed in the catalyst as reported in “Proceedings of the '09 Meeting and Workshop of Fuel Cell-Related Catalyst Division in Catalyst Society of Japan”.
JP 2009-173627A describes an electrode catalyst for fuel cells which is obtained by heating a mixture of a metal complex and an organic cyclic compound. JP 2009-234918A discloses a catalyst having a modified metal complex obtained by subjecting a mononuclear-metal complex having an aromatic skeleton to a modification treatment selected from a heat treatment, an irradiation treatment with a radiation, and a discharge treatment.
The problem of these processes is that the polymer material contains low-molecular impurities, such as a solvent, and needs removal of the impurities, which makes the production process complicated. In contrast, processes starting with a low molecular compound are advantageous in that the low molecular compound may previously be purified to provide a high purity precursor. This offers a broader choice of precursor materials.
Carbon alloys prepared from low molecular compounds include those obtained by baking a metal complex and an unsaturated compound as disclosed in JP 2009-39623A and those obtained by baking a metal complex and a polycyclic organic compound having at least two hetero atoms on the same ring as disclosed in JP 2010-270107A. However, metal complexes are difficult to purify, and the unsaturated compound is structurally limited because it must react with the ligand of the metal complex. Due to these problems it is not easy to stably prepare a carbon alloy of high oxygen reduction activity, and the advantage of using a low molecular compound is hardly achieved. “Jun Murayama, Carbon, 48, 3271 (2010)” reports preparation of a carbon alloy by baking a mixture of a nitrogen-containing polycyclic organic compound, ferric gluconate, and magnesium acetate, but the oxygen reduction activity of this carbon alloy catalyst is lower than that of the carbon alloys of polymer origin so that it is difficult to eliminate the need to use a noble metal such as platinum or palladium.
On the other hand, metal-free low molecular compounds are free of the problems, such as difficulty of purification and structural limitation. Using a metal-free low molecular compound, nevertheless, poses the following problem. “Jun Murayama and Ikuo Abe, Journal of the Electrochemical Society, 154(3), B297 (2007)”, for example, reports a case in which a mixture of an amino acid, glucose, and iron lactate is dehydrated and baked. The report reveals that the amino acid alone decomposes at 350° C. and cannot carbonize, indicating the difficulty in carbonizing a metal-free low molecular compound by baking. It has thus been believed by those skilled in the art that a carbon alloy is not obtained from a metal-free low molecular compound.